Reinforced hollow panel and method of making

ABSTRACT

A hollow upper deck panel for a plastic pallet has a smooth solid top surface and is made by thermoforming heated upper and lower twin sheets of thermoplastics material. The lower sheet is vacuum-formed into a lower mold section to form parallel spaced inverted U-shaped elongated open ribs projecting upwardly and having top portions fused to the upper sheet. The upper sheet is vacuum-formed into an upper mold section which has extendable and retractable blades to form parallel spaced elongated closed double wall ribs projecting downwardly between the open ribs and have bottom portions fused to the lower sheet. The closed double wall ribs extend transversely or perpendicular to the open ribs, and the ribs reinforce the deck panel in all directions. Support structure for the upper deck panel is also formed of plastics material and defines spaces for receiving parallel spaced forks of a forklift truck.

BACKGROUND OF THE INVENTION

In the manufacture of large pallets of rigid plastics material andhaving recesses or spaces for receiving parallel spaced forks of aforklift truck, it is well known to form the pallets with a twin sheetthermoformed upper deck, for example, as disclosed in U.S. Pat. No.5,197,396, No. 5,413,052, No. 5,676,064 and No. 5,813,355. It is alsoknown to make or form upper decks and other components for a plasticpallet by extruding a plastic parison and directing the parison intoblow molding equipment, for example, as disclosed in U.S. Pat. No.5,845,588, No. 6,962,115 and No. 6,976,437. Blow-molding processes havealso been used to form rigid hollow plastic panels, for example, asdisclosed in U.S. Pat. No. 6,826,887. Plastic pallets have also beenmade by using injection molded components, for example, as disclosed inU.S. Pat. No. 6,389,990, No. 6,840,181 and No. 7,197,989. It is furtherknown to make plastic pallets by rotational molding, for example, asdisclosed in U.S. Pat. No. 6,807,911.

In the manufacture of any plastic pallet or hollow plastic panel, it isdesirable to maximize the strength/weight ratio while minimizing thecost of manufacturing or producing the pallet or panel. Also, for manyuses of a plastic pallet or panel, it is desirable for the top surfaceto be substantially smooth and solid without openings which can trap orcollect undesirable debris or material. When the top deck and the bottomdeck of a plastic pallet are vacuum-formed by a twin-sheet thermoformingprocess, it is also known to insert tubular metal reinforcing membersbetween the sheets, for example, as disclosed in above mentioned U.S.Pat. No. 5,197,396 and No. 5,413,052. The top and bottom decks or panelsof a plastic pallet formed by a twin-sheet or triple sheet thermoformedprocess are also commonly reinforced by vacuum-forming integral hollowor open ribs in the top and/or bottom sheets, for example, as disclosedin above U.S. Pat. No. 5,676,064, No. 5,813,355 and No. 6,749,418.

SUMMARY OF THE INVENTION

The present invention is directed to an improved twin-sheet thermoformedplastic panel incorporating integral rib reinforcement whichsignificantly increases the strength of the panel and which also helpsto minimize the thicknesses of the twin-sheets forming the panel. Apanel constructed in accordance with the invention is ideally suited foruse as the top deck of a plastic pallet in that the panel has asubstantially solid top surface without any recesses or cavities whichcan collect dirt and debris.

In accordance with the illustrated embodiment of the invention, areinforced hollow panel is produced by a twin-sheet thermoformingprocess wherein a lower thermoplastic sheet is vacuum-formed into alower mold section to form parallel spaced inverted U-shaped elongatedopen ribs projecting upwardly and having spaced side walls. An uppersheet of thermoplastic material is vacuum-formed into an upper moldsection which has a set of extendable and retractable parallel blades toform parallel spaced elongated closed double wall ribs projectingdownwardly and having adjacent side walls. When the mold sections closeon the heated sheets, top portions of the open ribs in the lower sheetfuse to the upper sheet, and the double walled closed ribs projectdownwardly between the open ribs and have bottom portions fused to thelower sheet. Air pressure is introduced between the fused together upperand lower sheets, before the sheets solidify, and the blades areretracted so that the adjacent walls of the double walled closed ribsfuse together. The closed double walled ribs extend transversely orperpendicular to the open ribs, and the end portions of the doublewalled ribs fuse to the side walls of the open ribs, with the resultthat the hollow plastic panel is reinforced in all directions. When thehollow plastic panel is used for the upper deck panel of a pallet, thesupporting structure for the upper deck panel is also preferably formedof plastics material, and a lower deck panel may be formed in the samemanner as the upper deck panel.

Other features and advantages of the invention will be apparent from thefollowing description, the accompanying drawings and the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a plastic pallet having a reinforcedhollow plastic panel constructed in accordance with the invention andforming the upper deck of the pallet;

FIG. 2 is a perspective section of the hollow plastic panel, takengenerally on the line 2-2 of FIG. 1;

FIG. 3 is an enlarged perspective fragmentary section of a cornerportion of the plastic panel shown in FIG. 2;

FIG. 4 is a fragmentary perspective section of the final twin-sheetthermoformed panel shown in FIG. 1 and taken generally on the line 4-4of FIG. 3;

FIG. 5 is a fragmentary section of twin-sheet thermoforming tooling withthe mold sections in an open position;

FIG. 6 is an enlarged fragmentary section of the tooling shown in FIG.5;

FIG. 7 is a fragmentary vertical section of the tooling shown in FIGS. 5& 6 with the mold sections in a closed position, with rib forming bladesin an extended position; and

FIG. 8 is a fragmentary section similar to FIG. 7 and showing the ribforming blades in a retracted position.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 illustrates a plastic pallet 10 having a lower deck panel 12connected by a set of peripherally spaced hollow legs 14 to a hollowplastic panel 15 constructed in accordance with the invention andforming the upper deck for the pallet 10. As shown in FIGS. 5-8, thehollow panel 15 is formed by twin-sheet thermoforming method and toolingas disclosed, for example, in U.S. Pat. No. 5,843,366, the disclosure ofwhich is herein incorporated by reference. The hollow panel 15 is formedfrom an upper thermoplastic sheet 18 (FIG. 5) and a lower thermoplasticsheet 20 during the twin-sheet thermoforming operation. The heated lowersheet 20 is vacuum-formed into cavities 23 and 26 within a lower moldsection 30 supported by parallel spaced I-beams 32. As shown in FIGS. 2& 3, when the lower sheet 20 is vacuum-formed into the cavities 23 and26 and other cavities within the lower mold section 30, elongatedinverted U-shape bottom open ribs 35 and 37 are formed within the lowersheet along with a peripherally extending generally vertical wall 42(FIG. 6) extending to a peripherally extending and outwardly projectinghorizontal lip or flange 44.

The vacuum-forming of the lower sheet also forms a series ofperipherally spaced open bottom cavities 46 and 48 (FIGS. 2-4) withinthe lower sheet 20, and the cavities receive the upper end portions ofthe legs 14 when the lower deck panel 12 is later assembled to the upperdeck panel 15. The lower deck panel 12 is also vacuum-formed to havecorresponding cavities 46 & 48 to receive the lower end portions of thelegs 14 which are later attached or fused to the lower deck panel 12 andupper deck panel 15 during final assembly of the pallet.

Referring to FIG. 5, the twin-sheet thermoform tooling includes an uppermold section 55 having an upper mold plate 57 rigidly connected to alower mold plate 59 with the upper mold plate 57 supporting a fluidcylinder 62. The upper mold plate 57 and lower mold plate are supportedby parallel spaced I-beams 65 which are connected to a hydrauliccylinder (now shown) for vertical movement as a unit in a conventionalmanner. The lower mold plate 59 is also supported by a peripherallyextending frame members 67 and parallel spaced members or rails 69 eachhaving a passage 72 for receiving cooling fluid. A plate 74 ispositioned between the plates 57 and 59 and has slots for receiving therails 69. The plate 74 is connected to the piston of the fluid cylinder62 and is movable vertically in response to actuation of the cylinder 62between the upper plate 57 and lower plate 59. The plate 74 supports aset of parallel spaced thin blades 75 for movement between extendedpositions (FIGS. 5-7) projecting downwardly through corresponding slotswithin the plate 59 and retracted positions (FIG. 8) where the bottomends of the blades 75 are flush with the bottom surface of the plate 59.

During the twin-sheet thermoforming process, the heated upper sheet 18is vacuum-formed into a cavity 78 within the plate 59 to provide thesheet with a smooth flat top surface 80 (FIGS. 1 & 4) surrounding by aperipherally extending vertical wall 82 (FIG. 6) which continues to forma peripherally extending and outwardly projecting horizontal lip orflange 84. The blades 75 extend downwardly during the vacuum formingoperation so that the heated sheet 18 forms a double wall rib 90 (FIGS.5 & 6) around each of the blades 75. After the upper sheet 18 and thelower sheet 20 have been vacuum-formed and are still hot and notsolidified, the upper mold section 55 shifts downwardly and closes onthe lower mold section 30 (FIG. 7) when the peripheral flanges 44 & 84fuse together, the upper end portions of the open ribs 35 & 37 fuse tothe top sheet 18, and the lower end portions of the double wall ribs 90fuse to the lower sheet 35. Also, when the mold sections close, the topwalls of the cavities 46 & 48 fuse to the upper sheet 18.

The blades 75 are then retracted upwardly (FIG. 8), and the hollow spacebetween the sheets 18 and 20 and surrounding the parallel adjacent sidewalls of the ribs 90 is pressurized with air so that the adjacent sidewalls of each rib 90 are fused together, as shown in FIG. 8, to form theclosed and fused double wall ribs 90 shown in FIGS. 2, 3, 4 & 8. As alsoshown in FIGS. 2 & 3, the closed double wall ribs 90 extend transverselyor perpendicular to the open ribs 35, and the opposite end portions ofthe closed double wall ribs 90 are fused to the adjacent side walls ofthe open ribs 35. After the twin sheet panel 15 is formed by thetwin-sheet thermoforming process, and the panel 15 is removed from theopen mold sections 30 & 55, the flat top surface 80 of the upper panel18 is smooth and solid without any cavities. The surface 80 has onlylinear marks or lines 95 where the blades 75 have been retracted (FIG.8) to form the closed double wall ribs 90.

From the drawings in the above description, it is apparent that a hollowthermoformed plastic panel and a plastic pallet made from the panel,provide desirable features and advantages. As one important advantage,the arrangement of the downwardly projecting closed double wall ribs 90within the vacuum-formed upper sheet 18 of the panel 15 and the crossingopen ribs projecting upwardly from the vacuum-formed lower sheet 20 withthe ribs fused to the opposing sheets, along with the fused peripheralflanges, provides the panel 15 with substantial strength in alldirections while minimizing the weight of the panel. This feature isespecially desirable when the panel 15 is used to form the upper deck ofa plastic pallet as shown in FIG. 1. In addition, by forming the closeddouble wall ribs 90 within the vacuum-formed sheet 18, the top surfaceof the panel is essentially solid, which is especially desirable whenthe panel is used to form a plastic pallet. Also, by fusing the oppositeends of the double wall closed ribs 90 with the side walls of thevacuum-formed open ribs 35, the pattern of crossing connected ribs addsfurther to the strength of the hollow panel 15. In addition, byincreasing the strength of the hollow panel 15 in all directions, thethickness of the sheets 18 and 20 may be reduced, with the result thatthe weight of the entire hollow panel 15 is reduced.

While the form of thermoformed panel herein described and its method ofconstruction constitute a preferred embodiment of the invention, it isto be understood that the invention is not limited to this precise formof panel, and that changes may be made therein without departing fromthe scope and spirit of the invention as defined in the appended claims.

1. A hollow thermoformed plastic panel adapted for use in a plasticpallet, comprising a vacuum-formed upper sheet of rigid plasticsmaterial and a separate vacuum-formed lower sheet of rigid plasticsmaterial, said upper sheet and said lower sheet having correspondingperipheral edge portions fused together by heat and pressure, said uppersheet of said panel including a series of elongated closed double wallribs projecting downwardly to said lower sheet with each of said ribshaving fused together side walls and bottom portions fused to said lowersheet, and said closed double wall ribs in said upper sheet beingeffective to reinforce said hollow panel.
 2. A hollow thermoformedplastic panel adapted for use in a plastic pallet, comprising avacuum-formed upper sheet of rigid plastics material and a separatevacuum-formed lower sheet of rigid plastics material, said upper sheetand said lower sheet having corresponding peripheral edge portions fusedtogether by heat and pressure, said lower sheet of said panel includinga series of vacuum-formed parallel spaced inverted U-shape elongatedopen ribs projecting upwardly and having spaced side walls and topportions fused to said upper sheet to reinforce said panel in a firsthorizontal direction, said upper sheet of said panel including a seriesof elongated closed double wall ribs projecting downwardly between saidparallel spaced open ribs in said lower sheet with each of said ribshaving fused together side walls and bottom portions fused to said lowersheet, and said closed double wall ribs in said upper sheet extendingtransversely to said open ribs in said lower sheet to reinforce saidpanel in a second direction transverse to said first direction.
 3. Ahollow panel as defined in claim 2 wherein said closed double wall ribshave opposite end portions intersecting and fused to said side walls ofsaid open ribs.
 4. A hollow panel as defined in claim 2 wherein saidclosed double wall ribs extend perpendicular to said open ribs.
 5. Ahollow panel as defined in claim 2 wherein said upper sheet has asubstantially solid top surface with parallel spaced linear markscorresponding to the locations of said closed double wall ribs.
 6. Ahollow plastic panel forming an upper deck panel of a plastic palletcomprising a lower plastic support structure supporting said upper deckpanel and defining with said upper deck panel spaces adapted to receiveparallel spaced forks of a forklift truck, said upper deck panelincluding a vacuum-formed upper sheet of rigid plastics material and aseparate vacuum-formed lower sheet of rigid plastics material, saidupper sheet and said lower sheet having corresponding peripheral edgeportions fused together by heat and pressure, said lower sheet of saidupper deck panel including a series of vacuum-formed parallel spacedinverted U-shape elongated open ribs projecting upwardly and havingspaced side walls and top portions fused to said upper sheet toreinforce said upper deck panel in a first horizontal direction, saidupper sheet of said upper deck panel including a series of elongatedclosed double wall ribs projecting downwardly between said parallelspaced open ribs in said lower sheet and having fused together sidewalls and bottom portions fused to said lower sheet, and said closeddouble wall ribs in said upper sheet extending transversely to said openribs in said lower sheet to reinforce said upper deck panel in a seconddirection transverse to said first direction.
 7. A hollow plastic paneland pallet as defined in claim 6 wherein said closed double wall ribshave opposite end portions intersecting and fused to said side walls ofsaid open ribs.
 8. A hollow plastic panel and pallet as defined in claim6 wherein said closed double wall ribs extend perpendicular to said openribs.
 9. A hollow panel and plastic pallet as defined in claim 6 whereinsaid upper sheet has a substantially solid top surface with parallelspaced linear marks corresponding to the locations of said closed doublewall ribs.
 10. A hollow plastic panel and pallet as defined in claim 6wherein said lower sheet includes upwardly projecting peripherallyspaced portions having top walls fused to said upper sheet and sidewalls defining cavities for receiving leg members of said lower supportstructure.
 11. A method of producing a hollow thermoformed plastic paneladapted for use in a plastic pallet, comprising the steps ofvacuum-forming an upper sheet of rigid plastics material andvacuum-forming a separate lower sheet of rigid plastics material, fusingcorresponding peripheral edge portions of the upper sheet and the lowersheet together by heat and pressure, forming a series of downwardlyprojecting closed double wall ribs within the upper sheet with each ofthe ribs having adjacent side walls, and fusing together the adjacentside walls of each of the ribs and fusing bottom portions of the ribs tothe lower sheet with heat and pressure to reinforce the hollow panel.12. A method of producing a hollow thermoformed plastic panel adaptedfor use in a plastic pallet, comprising the steps of vacuum-forming anupper sheet of rigid plastics material and vacuum-forming a separatelower sheet of rigid plastics material, fusing corresponding peripheraledge portions of the upper sheet and the lower sheet together by heatand pressure, vacuum-forming a series of upwardly projecting parallelspaced inverted U-shape elongated open ribs within the lower sheet withthe open ribs having spaced side walls and top portions, fusing the topportions of the open ribs to the upper sheet to reinforce the panel in afirst horizontal direction, forming in the upper sheet a series ofelongated closed double wall ribs projecting downwardly between theparallel spaced open ribs in the lower sheet with each of the closedribs having adjacent side walls and a bottom portion, fusing theadjacent side walls of each of the closed ribs together and fusing thebottom portions of the closed ribs to the lower sheet, and positioningthe closed double wall ribs in the upper sheet transversely to the openribs in the lower sheet to reinforce the panel in a second directiontransverse to said first direction.
 13. A method as defined in claim 12including the step of fusing opposite end portions of each of the closeddouble wall ribs to the side walls of the open ribs.
 14. A method asdefined in claim 12 wherein the closed double wall ribs are extendedperpendicular to the open ribs.
 15. A method as defined in claim 12 andincluding the step of forming the upper sheet with a substantially solidtop surface having parallel spaced linear marks corresponding to thelocations of the closed double wall ribs.
 16. A method as defined inclaim 12 including the step of vacuum-forming the lower sheet withupwardly projecting peripherally spaced portions having top walls fusedto the upper sheet and side walls defining with the top walls downwardlyfacing open bottom cavities.